There is a need to accurately determine the three-dimensional position and velocity of an airborne threat. In this project, a special GPS/INS system is designed for providing highly accurate position and velocity information. In the system, several innovative GPS/INS integration techniques are employed: (1) Advanced real-time GPS attitude determination algorithms and integer ambiguity resolution approaches; (2) novel SDGPS (Sequential Differential GPS) and common-view GPS techniques for effectively eliminating GPS errors; (3) innovative GPS/INS structures for effectively integrating Global Positioning System (GPS) and Inertial Navigation System (INS) position (pseudorange), as well as attitude measurements. The advances of the project are meant to improve the performance of conventional GPS/INS integrated systems which may not be suitable for position determination purposes, because: (1) Their attitude accuracy, especially the INS azimuth accuracy, is still limited by INS inertial alignment errors and gyro drift errors; (2) the time-synchronization problem is not considered in conventional designs; (3) the techniques for eliminating the GPS errors that are common to various carrying platforms are not addressed in conventional designs.